Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L7/00—Arrangements for synchronising receiver with transmitter
  • H04L7/0075—Arrangements for synchronising receiver with transmitter with photonic or optical means
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/61—Coherent receivers
  • H04B10/616—Details of the electronic signal processing in coherent optical receivers
  • H04B10/6161—Compensation of chromatic dispersion
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J7/00—Measuring velocity of light
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
  • G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
  • G01M11/333—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face using modulated input signals
  • G01M11/334—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face using modulated input signals with light chopping means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
  • G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
  • G01M11/335—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face using two or more input wavelengths
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/61—Coherent receivers
  • H04B10/63—Homodyne, i.e. coherent receivers where the local oscillator is locked in frequency and phase to the carrier signal

Definitions

• the present invention relates to a method of controlling a timing state of a local oscillator being synchronized by a timing signal provided by means of one-way transmission from a remote master oscillator through a dielectric waveguide, to a time receiver, for providing a time indication based on a timing signal provided through one-way transmission from a remote master oscillator through a dielectric waveguide, and to a system for one-way time transmission.
• Time Absolute and coordinated time
• UTC Universal Time
• UTC(k) a physical realization of UTC
• UTC(k) a physical realization of UTC
• the transmission delay will typically vary. To achieve a correct transfer of Time, this variation in transmission delay must be compensated for.
• One method of handling the variation in transmission delay is to employ so-called two-way transfer.
• a signal indicative of the Time is returned from the user to the source of Time. Based on this signal, the source can determine the transmission delay, and send information about the
• this method requires two-way communication to each user that is connected to a particular source of Time, which may require extensive installation and limits the number of users that a certain source of Time can service.
• satellite signals from several GPS-satellites are used.
• many users do not have access to outdoor antennas.
• a general object of the present invention is to provide an improved transmission of absolute time, and in particular an improved way of handling variations in transmission delay between a source of Time and a user of Time.
• a method of controlling a timing state of a local oscillator being synchronized by a timing signal provided through one-way transmission from a remote master oscillator through a dielectric waveguide comprising the steps of: receiving a first timing signal modulated on a first electromagnetic carrier having a first carrier wavelength and transmitted through the dielectric waveguide; receiving a second timing signal modulated on a second electromagnetic carrier having a second carrier wavelength different from the first carrier wavelength and transmitted through the dielectric waveguide together with the first timing signal; providing a signal indicative of a difference between a transmission time of the first timing signal and a transmission time of the second timing signal through the dielectric waveguide; and controlling the timing state of the local oscillator based on the first timing signal and the signal indicative of the difference between the transmission time of the first timing signal and the transmission time of the second timing signal.
• steps may be performed at one point in time, and other steps at another point in time.
• the present invention is based on the realization that variations in transmission time through a dielectric waveguide can be deduced by studying the variation in the difference in transmission time between simultaneously transmitted signals modulated on electromagnetic carriers having different wavelengths. There is a one-to-one relationship between the change in transmission time from an initial transmission time to an instantaneous transmission time, and the change in wavelength-dependent transmission time difference from an initial wavelength-dependent transmission time difference to an instantaneous wavelength-dependent transmission time difference. Given that the transmission properties of the dielectric waveguide, such as the temperature dependence of the (wavelength-dependent) propagation velocity through the dielectric waveguide, are at least
• the above-mentioned change in transmission time can be deduced from a measured change in wavelength-dependent transmission time.
• the step of controlling the timing state may comprise the steps of: delaying the first timing signal based on the signal indicative of the difference between the transmission time of said first timing signal and the transmission time of said second timing signal to form a delayed first timing signal; and synchronizing the local oscillator using the delayed first timing signal.
• the delay may be either positive or negative.
• An instantaneous transmission time that is less than the initial transmission time is
• the step of controlling the timing state may comprise the steps of: synchronizing the local oscillator using the first timing signal; and controlling a frequency of the local oscillator using the signal indicative of the difference between the transmission time of said first timing signal and the transmission time of said second timing signal.
• the local oscillator may, for example, be a voltage controlled oscillator.
• the signal indicative of the wavelength-dependent difference in transmission time may then be a voltage signal, which may, directly or indirectly, be used to control the voltage controlled oscillator.
• the first timing signal and the second timing signal may be provided by the remote master oscillator.
• first timing signal and the second timing signal may be in phase when entering the dielectric waveguide.
• the first and second timing signal may be sine waves of a suitable frequency, such as 10 MHz.
• a time receiver for providing a time indication based on a timing signal provided through one-way transmission from a remote master oscillator through a dielectric waveguide
• the time receiver comprising: combining circuitry arranged to receive a first timing signal modulated on a first electromagnetic carrier having a first carrier wavelength and transmitted through the dielectric waveguide; and a second timing signal modulated on a second electromagnetic carrier having a second carrier wavelength different from the first carrier wavelength and transmitted through the dielectric waveguide together with the first timing signal, the combining circuitry being configured to output a signal indicative of a difference between a transmission time of the first timing signal and the second timing signal through the dielectric waveguide; and a local oscillator being controllable to provide the time indication based on the first timing signal and the signal indicative of the difference between the transmission time of the first timing signal and the second timing signal through the dielectric waveguide.
• the combining circuitry may advantageously comprise: a first sensor arranged and configured to convert the first timing signal to a first electrical timing signal; a second sensor arranged and configured to convert the second timing signal to a second electrical timing signal; and a mixer arranged to receive the first electrical timing signal and the second electrical timing signal and configured to output the signal indicative of the difference between the transmission time of the first timing signal and the second timing signal through the dielectric waveguide.
• the time receiver may further comprise delay circuitry arranged to receive the first timing signal and the signal indicative of the difference between the transmission time of the first timing signal and the second timing signal through the dielectric waveguide, and configured to adjust a delay of the first timing signal by a delay time based on the signal indicative of the difference.
• the local oscillator may be a controllable oscillator being arranged to be controlled using the signal indicative of the difference between the transmission time of the first timing signal and the second timing signal through the dielectric waveguide.
• the controllable oscillator may, for example, be a so-called voltage controllable oscillator.
• the first and second electromagnetic carriers may be optical carriers
• the dielectric waveguide may be an optical fiber
• time receiver may advantageously be included in a one-way time transmission system, further comprising a time transmitter comprising: a first source arranged to provide a first electromagnetic carrier having a first carrier wavelength; a second source arranged to provide a second electromagnetic carrier having a second carrier wavelength different from the first carrier wavelength; and a master oscillator arranged to control the first source and the second source to modulate a timing signal on the first electromagnetic carrier and on the second electromagnetic carrier.
• Fig 1 is a schematic illustration of a one-way time transmission system according to an example embodiment of the present invention
• Fig 2 is a flow chart schematically illustrating an embodiment of the method according to the present invention.
• Fig 3 is a diagram schematically illustrating the variation over time in transmission time of an electromagnetic signal through a dielectric
• Fig 4 schematically illustrates an additional embodiment of the time receiver according to the present invention. Detailed Description of an Example Embodiment of the Invention
• Fig 1 schematically shows a one-way time transmission system 1 according to an embodiment of the present invention, comprising a time transmitter 2 for transmitting timing signals, and a time receiver 3 for receiving the timing signals.
• the time transmitter 2 and the time receiver 3 are interconnected by a dielectric waveguide, here in the form of an optical fiber 4 having known transmission properties.
• the optical fiber 4 may have been previously characterized through testing, or material properties and
• Thin lines in fig 1 represent electrical interconnects, while bold lines represent optical fibers.
• the time transmitter 2 comprises a master clock 6, which is a highly accurate oscillator or signal generator that may be synchronized with a reference time, such as the Coordinated Universal Time that was mentioned in the Background section.
• the master clock 6 is arranged to control a first laser 7 and a second laser 8 to modulate the light output by the first 7 and second 8 lasers by the same constant frequency timing signal ST, which may, for example, be a 10 MHz signal.
• the first laser 7 is controllable to provide a first electromagnetic carrier in the form of light at a first wavelength ⁇ , for example 1310 nm
• the second laser 8 is controllable to provide a second electromagnetic carrier in the form of light at a second wavelength ⁇ 2 , for example 1550 nm.
• a first timing signal Sn modulated on the first carrier wavelength ⁇ and a second timing signal ST2 modulated on the second carrier wavelength ⁇ 2 are generated.
• the first STI and second ST2 timing signals are combined in a in a WDM (Wavelength Division Multiplexing
• the first Sn and second ST2 timing signals travel together through the optical fiber 4 until they reach the time receiver 3.
• the time receiver 3 comprises a WDM-splitter 1 1 for splitting incoming light by wavelength into the first STI and second ST 2 timing signals.
• the first timing signal STI is subsequently converted into a first electrical timing signal Snei by a first optoelectronic detector 12, and the second timing signal ST 2 is converted into a second electrical timing signal Sj 2 ei by a second
• the first electrical timing signal Snei is divided by divider circuitry, such as a WPD (Wilkinson Power Divider) 15.
• WPD Wideband Power Divider
• One portion of the first electrical timing signal Snei is provided to local timing circuitry 16, comprising a controllable local oscillator 17 (also referred to as "slave clock") and signal processing circuitry 18, and another portion of the first electrical timing signal Snei is provided to combining circuitry, here in the form of a mixer 20.
• the second electrical timing signal Si 2 ei- Output from the mixer 20 is a difference signal S d itf indicative of a difference ⁇ 2 between a transmission time ⁇ through the optical fiber 4 of the first timing signal STI and a transmission time ⁇ 2 through the optical fiber 4 of the second timing signal ST2-
• the difference signal Sditf is provided to the local timing
• the first electrical timing signal Snei and/or the difference signal S d itf is/are conditioned by the signal processing circuitry 18 to provide a compensation signal S ⁇ mp that can be used to control the controllable oscillator 17 to compensate for variations over time of the transmission delay caused by the transmission of the timing signals through the optical fiber 4.
• the signal processing circuitry 18 may be analog and/or digital circuitry that is configured to convert the difference signal S d itf to the compensation signal S ⁇ m P based on the relations provided below under the heading
• the relation between the compensation signal S ⁇ mp and the difference signal Sditf, and thus the configuration of the signal processing circuitry may advantageously be determined empirically, for example by correlating the difference signal S d itt with a measured value of the variations in transmission time for a certain period of time prior to installation/configuration of the one-way time
• a first timing signal Sn modulated on a first electromagnetic carrier having a first wavelength ⁇ and transmitted through a dielectric waveguide 4 is received, and a second timing signal ST2 modulated on a second electromagnetic carrier having a second wavelength ⁇ 2 different from the first wavelength ⁇ is received.
• the first Sn and second ST2 timing signals have been transmitted together through the same dielectric
• the first Sn and the second ST2 timing signals may be substantially in phase when entering the dielectric waveguide 4.
• step 102 a signal Sdiff indicative of a difference between a transmission time ⁇ of the first timing signal Sn and a
• the difference signal Sdiff may be provided by combining the first timing signal Sn and the second timing signal ST2 as was described above in connection with fig 1 .
• Other exemplary ways of providing the difference signal Sdiff will be described below with reference to fig 4, which schematically illustrates an additional embodiments of the time receiver according to the present invention.
• the difference signal Sdiff can be used to control the timing state of the local oscillator 17 on the receiver side 3 of the one-way time transmission system 1 .
• the variations in transmission delay through the dielectric waveguide 4 can be compensated for, given that transmission properties of the dielectric waveguide 4 are known and an initial transmission delay through the dielectric waveguide 4 is known.
• the timing state of the local oscillator 17 is thus controlled based on the first timing signal Sn and the difference signal Sdiff.
• the difference signal Sdiff may, for example, be used to delay the first timing signal STI such that a delayed version of the first timing signal is provided to the local oscillator 17.
• the difference signal Sdiff may be used to control the oscillation frequency of the local oscillator 17.
• fig 3 shows an exemplary diagram over the variations in transmission time ⁇ through the dielectric waveguide (4 in fig 1 ) over time.
• fig 3 shows an exemplary diagram over the variations in transmission time ⁇ through the dielectric waveguide (4 in fig 1 ) over time.
• the initial transmission time is determined to be ⁇ 0 . This may, for example, be upon installation of the time transmission system. As time passes,
• the instantaneous transmission time ⁇ therefore differs from the initial transmission time ⁇ 0 by the transmission time difference ⁇ .
• This transmission time difference ⁇ is derivable from the above-mentioned difference signal S d iff, as will be described below under the heading "Theory”.
• the receiver configuration schematically shown in fig 4 may be inserted after the first 12 and second 13 optoelectronic detectors in fig 1 , such that the input signals to the receiver configurations are Snei and Sj 2 ei as is indicated in fig 4.
• Fig 4 shows a time receiver 30 that differs from that in fig 1 in that the first timing signal Snei and the second timing signal Sj 2 ei are not directly mixed, but that combinations of a local oscillator signal SLO and the first Snei and second Sj 2 ei timing signals are mixed. Moreover, the first Snei and second Sj 2 ei timing signals are amplified.
• the time receiver 30 in fig 4 comprises a first amplifier 31 arranged to amplify the first timing signal Snei and a second amplifier 32 arranged to amplify the second timing signal Si 2 ei-
• the time receiver 30 further comprises a local oscillator 33 (in addition to the local oscillator 17 in the timing circuitry 16) and first 34 and second 35 mixers for mixing the signal SLO from the local oscillator 33 with the amplified first Snei and second Sj 2 ei timing signals.
• the time receiver in fig 4 comprises a third mixer 37 for combining the outputs from the first 34 and second mixers 35 as the difference signal S d iff.
• the theory for single way dual wavelength optical fiber time and frequency transfer is based on the transit time ⁇ for propagation of a single mode in an optical fiber expressed as the group velocity for a certain distance L and the wavelength ⁇ .
• n is the refractive index and c is the speed of light in vacuum.
• the transit time ⁇ sometimes known as the group delay time, in a fiber is thus dependent on the refractive index and the wavelength. This means that two different wavelengths will propagate at different velocity in the same fiber.
• a standard single mode fiber is dependent on environmental conditions, and previous studies have shown that temperature is the most important factor to include in the calculations. By calculating the derivative of the transit time with respect to temperature, both wavelength and refractive index will be taken into account as follows:
• wavelengths are influenced by temperature, and based on this, the variations in transfer time can be calculated.
• the time transfer technique uses the property that the variations are different, but correlated, which is also supported by experimental results.
• the refractive index of the fiber can be described by the so-called Sellmeier equation:
• the material dispersion can then be calculated as:
• the length of the optical fiber also varies. It is here assumed that the cabling or mounting will stretch the fiber at increasing temperature, however leaving the volume intact. The variations in dimensions of the glass is assumed to be negligible. If the core of the fiber is modeled as a glass cylinder, of length L and diameter d, a geometrical approach gives that the variation in temperature will change the length with AL(T-T 0 ) and the diameter with Ad(T-T 0 ), such that
• T is the temperature and T 0 is the reference temperature.
• n 2 is the refractive of the cladding and ⁇ is the relative difference of refractive index in the core and in the cladding.
• V and b are the normalized frequency and the normalized propagation constant, respectively, and can be found through:

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• 2009
  • 2009-11-12 SE SE0901439A patent/SE534634C2/sv unknown
• 2010
  • 2010-11-11 WO PCT/SE2010/051242 patent/WO2011059387A1/en active Application Filing
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